Exhaust gas filters and filtering systems for removing constituents from exhaust gases of internal combustion engines are known from US2005/0126139.
A carbon nanotube (CNT) consists purely of carbon and is basically a graphene sheet. The sheet is rolled into a tube to form a CNT. By choosing the rolling direction, CNTs with different electrical properties can be generated. The actual tubular arrangement together with the strong carbon-carbon bonds results in a mechanically, chemically and thermodynamically extremely robust structure. The tensile stress necessary to rupture a CNT exceeds that of the strongest steel. Thermodynamically, CNTs are stable in air to up to 750° C. CNTs interact and attract each other through Van der Waals forces that arise from temporary imbalances of the electron distribution within atoms. Through these attracting forces, several CNTs can be put together to form an array of CNTs or can be randomly orientated, thereby keeping their integrity without requiring any additional structural support.
The unique properties of CNTs can be used for highly efficient gas separation. CNTs can have single or multiple walls and can be generated with a defined diameter, which is the key element for gas separation. The separation can be based on adsorption and sieving.
Sieving exploits the variation of geometrical shape and size of molecules: Growing a CNT membrane on a suitable substrate allows the pore size of the membrane to be controlled so as to allow only molecules smaller than the pore diameter to pass. This type of control over the pore size is not possible when using conventional membranes based on polymers. A nano-scale filter using a carbon nanotube membrane is proposed in US Patent 2007/0051240, which discloses the use of a porous supporting component.
Experiments have shown that the transport of gases through CNT membranes is much faster than predicted by traditional continuum theory (Skoulidas et al., Phys. Rev. Lett. 89, 185901, 2002). For a range of different CNT membranes the flux was up 8,400 times higher than standard non-slip hydrodynamic flow as it governed the transport in polycarbonate membranes. This is likely caused by the intrinsic smoothness of the CNT walls for which the gas wall interactions are mainly reflective, i.e., collisions do not lose forward momentum and only partially diffusive (Knudsen model with partial slip). This phenomenon has been affirmed by molecular dynamics simulations.
Another major advantage of using CNT based filters over conventional membrane filters is the fact that they can be cleaned repeatedly after each filtration process thus regaining their full filtering efficiency. Because of their high thermal stability, CNT filters can be operated at temperatures of ˜400° C., which are several times higher than the highest operating temperatures of the conventional polymer membrane filters (˜52° C.) (Srivastava et al., Carbon nanotube filters, Nature Materials Letters, 3, 612, 2004).
Adsorption exploits the affinities of a molecule to CNT. Previous studies have investigated the CO2 adsorption in single-walled (Cinke et al., CO2 adsorption in single-walled carbon nanotubes (SWCNTs), Chemical Physics Letters, 376 (2003) 761-766) and multi-walled CNTs (Su et al., Capture of CO2 from flue gas via multi-walled carbon nanotubes (MWCNTs), Science of the Total Environment 407 (2009) 3017-3023).
Cinke et al. investigated the CO2 adsorption on HiPCo (high pressure CO disproportionation process) SWCNT in a temperature range of 0° C. to 200° C. SWCNTs adsorbed nearly twice the volume of CO2 compared to activated carbon. They performed experiments showing a CO2 heat of adsorption of 2303 J/mol (0.024 eV) in SWCNTs. They found that the adsorption is mainly a physisorption process1 and further confirmed through computations using second-order Möller-Plesset perturbation theory a similar binding energy showing that the CO2 is physisorbed side-on to the nanotube. Physisorption is due to van der Waals forces between adsorbent molecules and adsorbents, whereas chemisorption takes place due to chemical interactions between the adsorbent molecules and the surface functional groups of adsorbents.
Su et al. showed that the adsorption capacities of CO2 on CNTs and on modified CNTs via 3-aminopropyl-triethoxysilane (APTS) solution decreased with temperature indicating the exothermic nature of adsorption process and increased with water content in air at 0%-7%. They also confirmed that the mechanism of CO2 adsorption on CNTs and CNT(APTS) is a physisorption process. CNT(APTS) showed good adsorption performance of CO2 at 20° C. as compared to other carbon and silica adsorbents reported in the literature.
The present invention has as an objective the mitigation of at least one of the disadvantages of the gas filters discussed above.